From : http://www.ehow.com/how_8134546_turn-off-compatibility-ie8-browser.html
Open Internet Explorer and navigate to any website.
Look to the immediate right of the URL in the address bar. If you see an icon that looks like a broken piece of paper, then "Compatibility View" is an option. If the icon is shaded, then "Compatibility View" is on. If it appears unshaded, then "Compatibility View" is off, but you can click the button to toggle it on and off again.
Click "Page," located just to the right of the tabs. If a check-mark is present to the left of "Compatibility View" in the drop-down menu, then "Compatibility View" is on. If it is unchecked, then it is currently off.
Disable Compatibility View for All Websites
Launch Internet Explorer 8.
Click on the "Tools" menu and select the "Compatibility View Settings" option. This will open a window where you can customize the compatibility view feature.
Remove the check in the check box next to "Display all websites using Compatibility View" to turn off the feature.